As prior art relating to cooling systems for vehicle engines, there is one that is provided with a heater core for heating a passenger room (refer to Patent Document 1 for example). This is provided with a first coolant route for circulating coolant between a waste heat recovery unit and the heater core and a second coolant route for circulating coolant between the waste heat recovery unit and a water jacket of an engine.
Then, based on a detection value of a water-temperature sensor provided on the first coolant route, it is carried out to circulate the coolant in the first coolant route without circulating the coolant in the second coolant route when the coolant is relatively low in temperature, and to circulate the coolant also in the second coolant route when the coolant temperature rises.
By so doing, in the aforementioned prior art, when the coolant is heated, the coolant in the water jacket is not circulated through the heater core, and thus, the engine can be prevented from being overcooled. Further, because when the coolant is heated, a large quantity of coolant is not circulated through the heater core, a heater unit including the heater core can be enhanced in heating performance.